Chapter 3: Node Reference--The Annotated VRML97 Reference Manual

TECHNICAL NOTE: VRML 1.0 included special-purpose versions of Transform—Scale, Rotate, and Translate nodes—and the more general MatrixTransform node. The special-purpose nodes were dropped from VRML 2.0 because they are equivalent to a Transform node with some of its fields left as default values. If their absence bothers you, their prototype definitions are trivial. For example:

     PROTO Translate [ exposedField SFVec3f translation ] {       Transform { translation IS translation }     }

Dropping MatrixTransform was much more controversial. Allowing arbitrary matrix transformations was a very powerful feature that is almost impossible to support in its full generality. The arbitrary 4×4 matrix of the MatrixTransform allows specification of perspective transformations that have singularities and degenerate matrices that cannot be inverted, both of which cause major implementation headaches. Lighting operations, for example, typically rely on transforming normal vectors by the inverse transpose of the transformation matrix, which is a big problem if the matrix cannot be inverted. And picking operations (determining which geometry is underneath the pointing device) are best done by transforming a picking ray into the object's coordinate space, which, again, is impossible if there is a degenerate or perspective transformation in the transformation stack. No VRML 1.0 browser completely implements the MatrixTransform node.

Restricting the legal values of a MatrixTransform was suggested, but doing that makes MatrixTransform just another representation for the Transform node. Since that representation is both more verbose (16 numbers, four of which would always be (0, 0, 0, 1) versus the ten for a simple translate/rotate/scale Transform node), conversion back and forth between the two representations is possible (see the Graphics Gems series of books by Academic Press for several approaches). MatrixTransform was used in relatively few VRML 1.0 worlds, and one of the design goals for VRML 2.0 was minimalism. For all of these reasons, MatrixTransform is not part of the VRML 2.0 specification.

TIP: To use the Transform node properly, it is important to understand the order of the transformation operations as they accumulate. The first step is the center field. It translates the local origin of the object to a new position before all the other operations take place. It does not translate the object and will have no effect if no other operations are specified. Think of this operation as specifying the location of the object's center point to be used for subsequent operations (e.g., rotation). For example, the default Box node is centered at (0,0,0), and represents a cube that spans -1 to +1 along all three axes. In the following file excerpt, the Box is parented by a Transform that specifies a center of (-3,0,0) and a rotation of +180 degrees about the Z-axis of the modified center. The result is an upside-down box centered at (-6,0,0):

     Transform {       center -3 0 0        rotation 0 0 1 3.14       children Shape { geometry Box {} }     }

The second operation, in order, is the scaleOrientation. This operation is the most obscure and is rarely used. The scaleOrientation temporarily rotates the object's coordinate system (i.e., local origin) in preparation for the third operation, scale, and rotates back after the scale is performed. This is sometimes handy when you wish to scale your object along a direction that is not aligned with the object's local coordinate system (e.g., skewing). The fourth operation is rotation. It specifies an axis about which to rotate the object and the angle (in radians) to rotate. Remember that positive rotations produce counterclockwise rotations when viewed down the positive axis. This is sometimes referred to as the right-hand rule (see any computer graphics or VRML tutorial book for an explanation). The last operation is translation. It specifies a translation to be applied to the object. Remember that translation will occur along the local axes of the object's coordinate-system.

TIP: Another important concept to understand is the order in which nested Transforms operate. Within a single Transform the operation order occurs as described, but when a Transform parents another Transform, the lowest level Transform is applied first and each subsequent parent's operations are applied in "upward" order. For example, the following excerpt defines two Transforms, T1 and T2. The first Transform, T1, performs a translation and a scale operation, and has a child T2. The Transform T2 performs a scale and a rotation operation. Therefore, the order of operations is: T2 scale, T2 rotation, T1 scale, and finally T1 translation. It is important to notice that T1's scale operation scales the rotated object (and produces a skew):

DEF T1 Transform {  scale 1 2 1               # Stretch along Y  translation 0 0 -3        # Translate back in Z  children DEF T2 Transform {    scale  1 1 10           # Stretch in Z    rotation 1 0 0 0.785    # Rotate 45 degrees, X axis    children Shape { geometry Box {} }  }}

EXAMPLE (click to run): The following example illustrates the Transform node. The first Transform, T1, is the parent transformation for all subsequent objects in the file. The second Transform, T2, uses default values and is transformed by its parent's transformations. The third Transform, T3, specifies a new center point and a rotation about that center point. Note that these operations take place before the T1's scale and translation. The fourth Transform, T4, scales and translates the object, and of course is also transformed by T1.

#VRML V2.0 utf8DEF T1 Transform {      # Parent xform for entire file  translation 0 0 -100  # Xlates entire file down Z  scale 1 2 1           # Scales entire file in Y  children [    DEF T2 Transform {  # Default transform at origin      children Shape {        geometry Box {}        appearance Appearance {          material Material { diffuseColor 1 0 0 }    }}}    DEF T3 Transform {   # Re-centered and rotated      center -3 0 0      rotation 0 0 1 3.14      children Shape {        geometry Cone {}        appearance Appearance {          material Material { diffuseColor 0 1 0 }    }}}    DEF T4 Transform {   # Scaled (half) and xlated +X      scale 0.5 0.5 0.5      translation 3 0 0      children Shape {        geometry Cylinder {}        appearance Appearance {          material Material { diffuseColor 0 0 1 }    }}}  ]}

3.52 Transform